First, I’ve heard the Miller-Urey experiment called into question on several grounds. First, the experimental setup was not actually based on our best estimation of what chemical conditions on the early earth were like. Rather, it was based on what chemical conditions were needed in order to form amino acids. Scientists now believe that the early earth atmosphere was not dense in methane and ammonia, the chemicals used the Miller-Urey experiment. Rather, it was dense in sulfur compounds, carbon dioxide, and nitrogen. Is this true?
Second, they also claim that the conditions of the Miller-Urey experiment were not actually a simulation of natural processes. In particular, the experimental setting used a “cold trap” that artificially separated out different vapors. Without this device, all of the amino acids would have disintegrated immediately. Is this right or not?
Third, the self-replicating RNA enzyme that they designed at Scripps does not emerge from amino acids. It emerges from RNA, obviously. The New Scientist article doesn’t give the details, but I presume they formed it by joining together shorter molecules of RNA. Is there are evidence of RNA nucleotides forming by themselves from an early-Earth environment?
My question: why isn’t primative life constantly being created? i mean, evolution has advanced the DNA-we now have millions of animal and plant species. But why arent new primative life forms continuing to appear? are they out-competed by more advanced forms?
Say an asteroid strike blew a chunk of wet Mars into space, and that abiogenesis hd occurred there already. The only things making it alive here would be the most primitive of organisms. That hypothesis is testable - we need to find primitive life on Mars that uses DNA. I don’t think it very likely either, but if we find DNA-based life floating in space, panspermia would get some support. The question is about our planet - clearly abiogenesis happened somewhere.
I think the agenda for non-religious panspermists is that it is difficult for life to begin, and so it is more likely to come from someplace else. I think the evidence is moving in the other direction myself.
First off remember life evolves over millions of years. Recorded human history only goes back maybe 5,000 years and we have only been able to even spot such life in the last few decades.
Second the early earth was a far different place than it is today. Maybe conditions still exist somewhere (maybe mid-ocean volcanic vents) where stuff like this can happen (I have no clue) but regardless the same conditions that allowed this stuff to happen largely do not exist anymore.
And yeah, anything that does start to evolve in this manner has to out compete all the other stuff already out there. Chances are high that some bacteria or what-have-you will float along and eat or otherwise out-compete the new whatever-it-is.
All that said would we be able to tell if a newly discovered bacteria (and new ones are discovered all the time) evolved into existence yesterday rather than a million years ago? (Again I do not know)
First, the conditions are different, so anything generated now would not look the same. Second, any organic compounds regularly synthesized in nature would probably be eaten by some form of bacteria.
On preview, what Whack-a-Mole said.
On your first question: The experiment has been repeated with a wide range of possible atmospheric conditions.
Your second and third questions have similar answers. In both cases they are testing for what is possible, not what actually happened. They are also working in labs, not the world. In some conditions many or even most of the molecules may have disintegrated. Remember that the experiment took place in a box in a room over a short period of time. The real thing had a large number of varied environments over a large period of time. It may be that the in the wild the conditions to let the chemicals reach a stable steady state was rarer than the conditions to create them in the first place. It may also be that thousands of systems of replicating chemicals were formed and either destroyed or eaten over time. The bottom line is we will probably never know for sure what did happen, we can only try and isolate what is could have happened, and possibly what is most likely to have happened.
Jonathan
As far as I know, all RNA and DNA nucleobases have been recreated in the lab under various assumptions about the primordial atmosphere; see, for instance, the work of Joan Oró. Also, the criticism against the Miller-Urey experiment (which mainly concerns itself with whether or not the atmosphere back then was reducing) is largely derived from using present-day volcanic gases as a model for the early atmosphere, something which may not be correct, according to this study, where an attempt was made to model the atmosphere directly from present models of planetary accretion.
However, the best evidence, IMO, for the creation of amino acids in a natural way is simply that they’re found in meteorites – I can’t conceive of any other reason for them to be there if they’re not fairly regularly created in nature.
The common argument that I’ve heard goes like this. The Miller-Urey experiment in the 50’s, and since that time we’ve been hearing that all 20 amino acids in life forms were synthesized in conditions modeled after the early earth. (I was taught to take this experiment at face value, in school, in the 90’s.) Yet scientists have known that the conditions in the original Miller-Urey experiment were not a good simulation of early Earth.
So they repeated the experiment with a more realistic set of conditions, and what did they find? They generally say that the realistic versions of the experiment produced “organic molecules”, while not mentioning that they’re no longer producing the amino acids that caused such a stir in the 50’s. My 10th grade chemistry students could explain that not all organic molecules have any relationship to living things. In fact, I’ve heard that the “organic molecules” they’ve produced in recent versions of the experiment are cyanide, formaldehyde, and alcohol. Needless to say, these are all hostile to life.
Is this true, or is there reason to believe that amino acids formed on the early earth?
An editor for Nature NY that I know sent me the following email on Dec. 22. It either has already appeared or is about to, so I guess the confidentiality is off:
Proof of abiogenesis would be hard to imagine, but disproof would be trivial: a god would have to appear and say, “I did it.” It is possible that it happened elsewhere (Mars?) and got to earth riding a stone dislodged by a meteor, but so what?
Except that’s not the question posed by the OP: “Is there any scientific evidence that abiogenesis occurred on our planet?
I am wondering if there is any scientific evidence that abiogenesis occurred on planet earth, at some stage in its history.”
The short answer is no. Yes, it seems logical that barring a Deity, abiogenesis must have occured* somewhere.* But it need not have occured here.
There is some evidence it could have occured here.
I am a little confused by this statement – the precise conditions on the early Earth are a matter of some debate now, and even more so back in the fifties, so how could they have known that the conditions used in the experiment weren’t a good model? As far as I know, atmospheric conditions as used by Miller and Urey are even today a contender for the primordial atmosphere, along with the volcanic gas-based CO[sub]2[/sub]/nitrogen rich model, in which amino acid synthesis has also been shown to occur, by the way.
And as for cyanide: even though it’s harmful to complex life today, actually one of the greatest breakthroughs in studying possible chemical reaction channels for abiogenesis was the prebiotic synthesis of adenine (the ‘A’ in the four ‘letters’ of the DNA, C, G, A, and T) from hydrogen cyanide by the aforementioned Joan Oró, so what’s harmful now way well have been hugely beneficial back then.
DrDeth, that was the question I was trying to address in my post, which was the one right after the OP, and before it headed in a material vs. supernatural direction. I had assumed that the “on our planet” was the key, as opposed to happening somewhere else and then getting here somehow.
But I disagree with you somewhat that we have no evidence, we just don’t have sufficient evidence. The fact that we know that the chemical building blocks were here already, and that we can estimate the likelihood of surviving self-replicating stuff to get ejected from somewhere else and survive the trip to Earth, then find Earth accommodating, those seem to count as “scientific” pieces of evidence to me.
This is the part that I don’t get, though. Maybe I’m just missing a piece of the puzzle, which is a strong possibility. But it was difficult for life to start on Earth, therefore it’s more likely that it started somewhere more hospitable, like… Mars?
Why does God love Mars more than He loves us? What in blazes is so special about Mars?
Well; so far all known life appears to have the same genetic heritage. That implies a single origin; most likely it’s a matter of life having such a major advantage over not-quite-life that the latter just gets eaten before it can establish itself.
As for whether or not life started on Earth; at the moment it’s scientifically most plausible, given Occam’s Razor. We’d have to go out into space and look to find out otherwise, probably. For example, if we find life and Mars and it has the same genetic heritage as ours, that implies life spread between planets - in which direction is another question. If we find life on Mars has multiple genetic heritages including ours, that would be pretty good evidence that Earth life came from a single source of contamination from Mars. And if we found life there with an obviously different genetic ancestry than ours, that would be good evidence that it originated separately on both planets.
And if we discovered the remains of an alien spacecraft dating from so long ago somewhere in the solar system, THAT will make the question something that would likely require interstellar travel to answer.
It is not more likely, it is just possible. I never said it was more likely, I am sure I did not. I was just answering the Op without making assumptions about the point they were trying to make.
I just used Mars as an example. I think the hypothesis is that the seeds, as it were, could have drifted over interstellar distances given enough time - which they had. Thus a small number of abiogenesis events could seed a large number of planets. Abiogenesis occurring on any planet in the solar system does not support it being a low probability event. If we found remains of Martian life clearly not related to us, that would indicate it is a high probability event, I think.
Actually the article you’ve cited below dismisses the methane and ammonia atmosphere, as does everything else that I’ve read on the topic. I just borrowed a book from the biology room next door and it implies widespread agreement on carbon dioxide and nitrogen, and also that such an atmosphere does not produce amino acids.
I’m aware that the atmosphere might not have been uniform and that there might have been pockets here and there that did match up with the conditions in the original experiment. That’s a possibility. I just think that we shouldn’t say that the experiment imitated conditions on early earth, when it isn’t true. We should be honest with students when we teach them stuff.
That’s definitely a worthwhile result. I wish it went into more detail about how they knew about iron and carbonates on early earth, and exactly what was produced. “Amino acids” is a huge class of compounds. I found a presentation on the guy’s webpage at UCSD which seems to say, if I’m interpreting it correctly, that his new experiment produced most, but not all, of the 20 amino acids in modern lifeforms.
Thanks for alerting me to that piece of information; I was totally unaware of that. It does seem, however, that not all the amino acids necessary for life are found in meteorites. Only eight of the twenty have been found so far.
More importantly, figuring out that model is only useful in terms of that it is likely to be a successful atmosphere in which to create life. Performing abiogenesis in an atmosphere entirely unlike that which existed on earth still proves abiogenesis is possible. There needn’t be only one path that leads to life. Almost definitely there is more than one.